This invention relates to optical temperature sensors.
Optical temperature sensors are used to provide an indication of the temperature of, for example, gas-turbine engines, where an optical output is required rather than an electrical output, or where an electrical sensor will not withstand the environment being measured.
At high temperatures, it is usual to use sapphire or similar high-temperature resistant, infra-red transparent materials to form a radiation guide between the sensing tip of the apparatus and a cooler region, where the sapphire guide is connected to a fibre-optic cable that is resistant only to lower temperatures. Radiation emitted by an emissive surface close to the tip of the sapphire guide falls on the guide and is supplied via the fibre-optic cable to one or more detectors which provide an electrical output to a processor. The processor provides an output representative of temperature to a display, a control or other utilization means.
Such sensors may be subject to considerable variations in temperature along the length of the sapphire guide, such as from 1300 degrees Centigrade at the sensing tip to 200 degrees Centigrade where the sapphire guide joins with the fibre-optic cable. The tip of the sensor may, furthermore, in some applications, be exposed to a lower temperature than some other part of the sensor, to the rear of the tip. This causes problems with conventional sensors because the sapphire itself has a thermal emissivity and will emit radiation into the guide with a spectrum dependent on its temperature. This can cause temperature measurement errors which are a particular problem where the sensor is required to measure accurately the temperature of the region at the tip of the sensor.